In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process and operably coupled to control circuitry by a process control loop.
Some field devices include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, and the like. Typically, each field device also includes communication circuitry that is used for communicating with a process control room, or other circuitry, over a process control loop. In some installations, the process control loop is also used to deliver a regulated current and/or voltage to the field device for powering the field device. The process control loop also carries data, either in an analog or digital format.
Traditionally, analog field devices have been connected to the control room by two-wire process control current loops, with each device connected to the control room by a single two-wire control loop. Typically, a voltage differential is maintained between the two wires within a range of voltages from 12-45 volts for analog mode and 9-50 volts for digital mode. Some analog field devices transmit a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. Other analog field devices can perform an action under the control of the control room by controlling the magnitude of the current through the loop. In addition to, or in the alternative, the process control loop can carry digital signals used for communication with field devices. Digital communication allows a much larger degree of communication than analog communication. Field devices that communicate digitally can respond to and communicate selectively with the control room and/or other field devices. Further, such devices can provide additional signaling such as diagnostics and/or alarms.
In some installations, wireless technologies have begun to be used to communicate with field devices. While many such wireless technologies have generally been developed to provide inexpensive data communication, such technologies have not been adapted for high reliability data communication. In distinct contrast, field devices sometimes provide crucial information required for effective control or monitoring of a given process. Should the process information not be communicated effectively, the process itself could degrade, or need to be taken completely offline for maintenance.